Depending on the process, purging methods can vary. Extrusion, unlike injection molding, is a continuous process. As a result, purging procedures should be performed regularly to avoid issues with product waste, untimely breakdowns and production downtime.
Long runs and the high output of extruders can result in an increased scrap rate on extended changeovers with long periods of dead time in between production runs. There are a range of purging compounds available today that focus on performance and are engineered to combat the challenges that extruders face.
For efficient purging, shops require a product and process that is quick and thorough, while avoiding costly damage to valuable machinery. Using the wrong chemicals or tools can damage screws and lead to production issues.
Many shops rely on a resin-based purging compound containing no harmful chemicals.
For extrusion purging, operators should ensure that all zones are in the proper temperature range for the type and grade purging compound being used.
The barrel should be run empty, and the hopper and feed throat cleaned and wiped. For some unfilled purging compounds, screens and dies can be left in place. Processors should check with a consultant from the maker of the compound being used for mesh size and minimum-required die clearances.
If the machine is heavily contaminated and using glass-filled grade (EX, PX and NF), a processor should remove the screen pack to avoid plugging. The processor may need to remove the die.
Operators can then feed one or two barrel capacities of purging material into the extruder. The operator should start purging at low screw speed. After material begins coming out from the head, the screw speed should be increased to the maximum safe level. Periodically, stop the screw and allow the material to sit for a few minutes. Resume the screw rotation at the maximum screw speed.
After continuing this process until most of the compound is purged from the barrel, the pile should be inspected. The extruder is clean and the process complete when the purging compound coming from the machine is free of contaminants.
It is worth noting that lower viscosity can be used to flush out corners, while higher viscosity to push out easy-flow melt.
A popular option for extruders today is single-dose compounds that allow operators to purge their machinery simply by dropping the packets into the feed throat/hopper of their machine.